1. Field of the Invention
The invention relates to an improvement of a photoconductor coupled liquid crystal light valve utilizing photoconductive effect of a photoconductive layer and electro-optical effect of liquid crystals.
2. Description of the Related Art
In recent years, as a memory device for optical information. a photoconductor coupled liquid crystal light valve utilizing photoconductive effect of a photoconductive layer and electro-optical effect of liquid crystals has been given attention. The photoconductor coupled liquid crystal light valve comprises basically photoconductive layers, a reflection layer, a liquid crystal layer and transparent electrodes sandwiching these layers. Basic function of this device will be detailed hereunder. Voltage is applied to the photoconductive layer and the liquid crystal layer from an outside power source through the transparent electrodes, so that the photoconductive layer and the liquid crystal layer are imposed with the voltages in proportion to electro-resistance of the layers, respectively. When light is riot applied to the device, since electro-resistance of the photoconductive layer is high, the voltage applied to the photoconductive layer and the liquid crystal layer is imposed substantially only on the photoconductive layer while the voltage given to the liquid crystal layer is lower, so that the liquid crystal layer does not produce electro-optical effect. When the light is applied to the device so to write information, electro-resistance of the photoconductive layer decreases, thereby causing to change voltage allocation to the photoconductive layer and the liquid crystal layer. That is, the photoconductive layer which decreases electro-resistance is only applied with lower voltage while the voltage imposed on the liquid crystal layer is higher than the threshold voltage so to bring the electro-optical effect. Hence, when the light contains image information and is applied to the photoconductive layer, it can generate an image pattern on the liquid crystal layer. FIG. 2 (A) shows the state of electric charge on the surfaces of the photoconductive layer 1 and the liquid crystal layer 2 when the device is not applied with light, and FIG. 2(B) that when the right half of the device is applied with light.
In a photoconductor coupled liquid crystal light valve using a material including a photoconductive layer for recording an image pattern, when the light is applied to the photoconductive layer so to write an image, it makes excitation and electric charge (carriers) is generated. The carriers naturally move to an interface between the liquid crystal layer and the photoconductive layer and tend to further shift laterally, thereby causing such problems as deterioration of resolution, deteriorating of image density and partial fragment or stain of images. In detail, when the carriers in the region A at the irradiated right half of the device in FIG. 2(B) partially shift to the non-irradiated left side region B, the region A shortens in the carriers to cause the liquid crystal layer to be subjected to weak electric field, thereby lowering the electro-optical effect, while the liquid crystal layer in the region B is subjected to higher electric field to produce feeble electro-optical effect. As a result, there occurs difference of image density between the regions A and B, or partial fragment of images, leading to an image of poor resolution.
Utilization of amorphous silicon which may contain hydrogen (called hereunder amorphous silicon or a-Si) for the photoconductive layer has been hitherto proposed in such official gazettes as of Japanese Unexamined Patent Applications Nos, 58-34435(1983), 58-34436(1983), 58-199327(1983), 59-81627(1984) and 59-170820(1984). The publications however do not at all cover prevention of the lateral shift of the carriers on the surface of the photoconductive layer in the device.
The present invention has been designed to overcome the above problem. An object of the invention is to provide a photoconductor coupled liquid crystal light valve in which electric charge on the surface of the photoconductive layer prevented from shifting laterally, thereby keeping the excellent image quality.